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S18 PH 121 chapter 5

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by

Richard Datwyler

on 11 October 2018

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Transcript of S18 PH 121 chapter 5

Forces!!
What are some classic examples of forces?
4 main types of forces
Gravity
Electromagnetic
Strong nuclear
Weak nuclear
How many forces are
acting on this frog
A. 1
B. 2
C. 3
D. 4
E. 5
"Could you explain more for me the correlation between mass and inertia?"
"What is a free body diagram?"
What forces are acting on a ball after it has been kicked and is rolling across the floor?
What forces are there on a car parked on a hill?
A jet is accelerating down the run way, what forces are acting on it? (don't neglect air resistance)
Mass is not weight, it is inertia
it is a resistance of motion for an applied force
Before we dive into F=ma
let us talk about kinematics and dynamics
this law relates the two.

The dynamics is what causes the motion

The kinematics is the resultant motion.
When something is moving it doesn't have multiple
accelerations. It has just one acceleration.

It can have multiple forces, but it will only have
one net or resultant force.
This distinction can help us as we add up all the forces.

If we find the direction of the one acceleration it will make solving the problems easier.
Newton's Second Law





Catch phrase:
F=ma
Stated: The acceleration of an object is directly
proportional to the net force acting on it, and
inversely proportional to its mass.
Nicer statement:
Are these the same?
Mathematically , Physically?
A Force is an action that can accelerate an object

Its SI units are Netwons , N

The base unit of a N comes from this 2nd law of motion

F =ma [N] = [kg][m/s ]
2
What net force would be required to bring a 2000 kg truck from rest up to 25 m/s in a time of 50 seconds?
Notice it is a vector equation
Thus it can be broken into components.

Also notice the sum, this means we have to
ADD all the different vectors together.
This is most effectively done in their individual components.
Stated:
Every object continues in its state of rest, or
uniform velocity in a straight line, as long as
no net force acts on it.
Newton's First Law of Motion
Catch phrase:
objects in motion stay in motion,
objects as rest stay at rest
Newton's First law is also known as
'the law of inertia'
What is inertia?
Mass is the measure of inertia.

The more mass something has
the more force is required to give it
some motion.

SI units : Kilogram , kg.
Is mass the same as weight?
Free-Body Diagrams
THIS IS KEY
Key parts to FBD

A. Particle
B. Coordinate system
C. All forces, note net force not on particle
Box pushed across a flat surface.
Car parked on a hill?
Bicycle picking up speed as it goes down a
hill.
Car going the same speed around a turn.
A cannon ball flying through the air (with no air resistance.)
A skydiver falling, and eventually reaching terminal velocity.
Draw FBD of the following
"I would like a better understanding of drag and thrust."
"Why does friction depend on the normal force? I also didn't understand too much the concept of inertia (with weight for example)."
Full transcript